Sintered abrasive particles and abrasive articles including them are useful for abrading, finishing, or grinding a wide variety of materials and surfaces in the manufacturing of goods. Of the wide variety of known abrasive particles, fused abrasive particles (e.g., including fused alumina, heat treated fused alumina, and fused alumina zirconia) and sintered ceramic abrasive particles (including sol-gel-derived sintered ceramic abrasive particles) are widely used in the abrasives art.
Alpha alumina abrasive particles are a major class of abrasive particles used in the abrasives industry. Fused alpha alumina abrasive particles are typically made by charging a furnace with an alumina source (such as aluminum ore or bauxite), as well as other desired additives, heating the material above its melting point, cooling the melt to provide a solidified mass, crushing the solidified mass into particles, and then screening and grading the particles to provide the desired abrasive particle size distribution. Although fused alpha alumina abrasive particles and fused alumina-zirconia abrasive particles are still widely used in abrading applications (including those utilizing coated and bonded abrasive products), the premier abrasive particles for many abrading applications since about the mid-1980s are sol-gel-derived alpha alumina particles (also referred to as sintered ceramic alpha alumina particles).
Sol-gel-derived alpha alumina abrasive particles may have a microstructure made up of very fine alpha alumina crystallites (also known as “alpha alumina crystal grains”), with or without the presence of secondary phases added. Sol-gel-derived alumina abrasives are conventionally produced by drying an aqueous sol or gel of an alpha alumina precursor (typically, but not necessarily, boehmite) to remove the water component of the gel, breaking up the dried gel into particles of the desired size for abrasive grits; optionally calcining the particles (typically at a temperature of from about 400-800° C.) to form a transitional alumina (e.g., gamma alumina), and then sintering the dried and optionally calcined particles at a temperature sufficiently high to convert them to the alpha alumina form.
In one embodiment of a sol-gel process, the alpha alumina precursor is “seeded” with a material having the same crystal structure as, and lattice parameters as close as possible to, those of alpha alumina itself. The “seed” (a nucleating agent) is added in as finely divided form as possible and is dispersed uniformly throughout the sol or gel. It can be added for the beginning or formed in situ. The function of the seed is to cause the transformation to the alpha form to occur uniformly throughout the precursor at a lower temperature than is needed in the absence of the seed. This seeded process produces a crystalline structure in which individual alpha alumina crystal grains (that is, those areas of substantially the same crystallographic orientation separated from adjacent crystals by high angle grain boundaries), are very uniform in size and are essentially all sub-micron in diameter. Suitable seeds include alpha alumina itself and other compounds such as alpha ferric oxide, chromium suboxide, nickel titanate, and other compounds that have lattice parameters sufficiently similar to those of alpha alumina to be effective to cause the generation of alpha alumina from a precursor at a temperature below that at which the conversion normally occurs in the absence of such seed.
Similarly, one or more alpha alumina crystal grain size modifiers (e.g., a spinel forming metal oxide) may also be added to the sol-gel, or impregnated into the dried sol-gel particles or calcined particles to control the size of alumina crystal grains in the resultant alpha alumina abrasive particle.
However, the use of seeds and/or alpha alumina crystal grain size modifiers adds complexity and cost to the sol-gel process.